Known types of electrolysis include alkali chloride electrolysis such as brine electrolysis, and alkali metal aqueous solution electrolysis such as alkaline water electrolysis and alkali sulfate electrolysis. In an alkali metal aqueous solution electrolytic apparatus, the electrolyzer houses a plurality of internal electrolytic cells. Each electrolytic cell has a cathode chamber that houses a cathode, an anode chamber that houses an anode, and a partition wall that separates the cathode chamber and the anode chamber. The inside of the electrolyzer is arranged so that the cathode chamber and anode chamber of adjacent electrolytic cells oppose one another, and a diaphragm is disposed between the electrolytic cells. For example, in a brine electrolytic apparatus, an ion exchange membrane method that uses an electrolyzer containing an ion exchange membrane as the diaphragm is often used (Patent Document 1).
In electrolysis using the type of electrolyzer described above, if operation of the electrolyzer is stopped due to trouble or the like, then a reverse current (a current in the reverse direction to the electrolytic current) flows through the electrolyzer. Particularly in the case of bipolar electrolyzers, which are the most common form of electrolyzers for brine electrolysis, the size of the reverse current increases in proportion to the square of the number of electrolytic cells. In recent years, there is a trend toward larger electrolyzers, resulting in an associated increase in the number of electrolytic cells. As a result, the size of the reverse current that flows when the electrolysis is stopped has also increased.
As a result of this reverse current flow, cathode degradation occurs in which the cathode catalyst (noble metal material) is eluted as a result of oxidation. In recent years, rather than platinum (Pt) or rhodium (Rh), the less expensive ruthenium (Ru) has become more widely used as the cathode catalyst material. However, because Ru is easily eluted as a result of reverse current, a more effective countermeasure for preventing oxidation caused by reverse current is required.
In order to prevent cathode degradation caused by reverse current, one measure that has been taken involves applying a very weak current through the electrolyzer during shutdowns of the electrolyzer, thereby maintaining the cathode potential at the hydrogen generation potential. However, there is a risk that the generated hydrogen may pass through the diaphragm, diffuse into the anode side, and mix with the oxygen gas generated at the anode side to form an explosive gas, and this risk must be avoided. Accordingly, initial capital costs and operating costs tend to increase as a result of the complexity of the operating procedure and the requirement for additional equipment.
Another countermeasure that has been proposed for suppressing cathode degradation caused by reverse current flow during operational shutdowns involves placing a material containing a substance that preferentially absorbs the reverse current inside the cathode chamber.
Patent Document 1 proposes providing a reverse current absorption layer, which is connected electrically to the cathode, inside the cathode chamber. The reverse current absorption layer in Patent Document 1 contains a material having a lower redox potential than the cathode material. Because the reverse current is consumed by an oxidation reaction of the reverse current absorption layer rather than the cathode, oxidative degradation of the cathode due to the reverse current is suppressed. The reverse current absorption layer of Patent Document 1 is formed by a deposition technique such as thermal spraying onto a substrate such as a current collector, metal elastic body or partition wall inside the electrolytic cell. Alternatively, a reverse current absorption body composed of a reverse current absorption layer formed on a separate independent substrate may be attached to an electrolytic cell component such as a current collector or metal elastic body.
Patent Document 2 proposes a cathode structure including an active cathode, a cathode current collector and an elastic cushion material, wherein at least a surface layer of the cathode current collector is composed of an active material that can consume a greater oxidation current per unit of surface area than the active cathode. Specific examples of this type of active material include Raney nickel, Raney nickel alloys, activated carbon-nickel composite plating, and composite plating of hydrogen storage alloy particles. When the electrolyzer is stopped and a reverse current flows, this active material on the cathode current collector preferentially consumes the oxidation current, enabling oxidation of the active cathode that accompanies anodic polarization to be suppressed to a minimum.